Carbon formed by pyrolysis of bone, wood, coal, sugar and other biological or organic materials and having an extended surface has long been known as a potent adsorbent. However, such carbon possesses undesirable properties for chromatography, giving poor symmetry of chromatographic peaks and slow mass transfer. These undesirable features arise from the presence of an uncontrolled variety of functional groups on the surface of the carbon, from the presence of impurities and from the presence of pores of a wide range of sizes including pores of very small diameter.
Three methods have recently been proposed for the production of porous carbon or equivalent material with improved chromatographic properties:
(A) Aggregates made from colloidal carbon blacks are given structural strength by deposition of pyrolytic carbon thereon. In this proposal the aggregates of carbon black are heated to a high temperature (typically 900.degree. C.) in a stream of inert gas (typically nitrogen) containing a pyrolysable organic vapour (typically benzene at, say, 20% mole fraction). The materials so formed have surface areas of 100 to 400 m.sup.2 g.sup.-1 and show good chromatographic properties, in particular reduced plate heights of less than 3 under optimum conditions of flow rate. However, the production of the aggregates is tedious and until hardened by deposition of pyrolytic carbon they are extremely fragile. This proposed method therefore does not appear to be suitable for large scale production.
(B) Silica gels with good pore structure are submitted to a treatment, such as that described in (A) above for carbon black aggregates, to produce a silica gel the internal surface of which is covered by a layer of pyrolytic carbon. This product gives a good chromatographic performance but the presence of a silica core is a disadvantage in that eluents aggressive to silica cannot be used: furthermore it is difficult to attain complete coverage of the silica surface by carbon because the carbon tends to deposit on surface nuclei already present rather than on the bare silica.
(C) A natural carbon formed, say, by pyrolysis of coal is treated with strong chemical reagents to remove most impurities and then heated to about 3000.degree. C. whereupon evaporation of the remaining impurities, and graphitization, occurs. Although the material before graphitization may have a surface area in the range 50 to 100 m.sup.2 g.sup.-, this is reduced to 5 to 10 m.sup.2 g.sup.-1 after graphitization. The chromatographic performance of this product to date has proved disappointing.
There thus remains a need for a method which will produce a porous carbon product of uniform surface properties, which has a good pore structure, uniform pore sizes and adequate mechanical strength. The present invention seeks to provide such a method which is distinguished from any of the above-described prior art methods.